Typical control systems for aircraft flight control surfaces use redundant hydraulic actuators to achieve the necessary reliability. Redundancy, is obtained by either using a plurality of actuators or coupling hydraulic cylinders in tanden with a common piston rod. Most prior methods use one or more hydraulically actuated linear spool type servo valves to control the flow of hydraulic fluid to the actuator. Thus, when the pilot of the aircraft moves the appropriate control in the flight station, a valve coupled to the control by the mechanical linkage is opened and hydraulic fluid, via a line coupled to the servol valve drives the spool, allowing a second source of hydraulic fluid to flow to the actuator. Such systems are heavy because of the necessity of having hydraulic lines running to the plurality of servo valves as well as hydraulic lines for driving the actuators.
In the so-called "fly-by-wire systems", when the pilot moves the controls, an electric signal is generated which actuates electrical/hydraulic valves mounted in proximity to the primary servo valves. Obviously, such a system is lighter in weight since the mechanical linkage system to the flight station is eliminated. Typical systems using this approach are disclosed in U.S. Pat. Nos. 3,338,138, Redundant Control System by Wood, and 3,543,641 Control for Spoilers And Like Aerodynamic Actuators of Aircraft by H. Deplante, et. al. The disadvantage of such systems is that to obtain the required reliability, the systems become very complex, large in size, and expensive to manufacture and maintain. For example, the U.S. Air Force F-16 aircraft requires three electrical mechanical actuators driving three separate linear spool type servo valves, with three monitoring valves and mechanical feedback linkage, which in turn control a single tandem actuator. The space shuttle is even more complex in that it uses a quad-redundant system.
With the advent of highly reliable electronic systems, and in particular the development of digital electronic components and circuits, along with advanced magnetic materials, it has become possible to use what is commonly called "direct drive systems". In such systems, the servo valves used to control the actuators are directly coupled to electromechanical drivers; for example, permanent magnet touque motors. An example of this type of system can be found in U.S. Pat. No. 2,826,896, Manually Controlled Electro-hydraulic System For Aircraft by S. G. Glaze, et. al. Glaze uses two hydraulic cylinders in tandem coupled by a common output shaft. The two servo valves are coupled to each cylinder and a torque motor is coupled to the ends of the spools by means of a mechanical linkage system. This system has several drawbacks. Because the spools are not mechanically coupled to each other, each torque motor has to be sized sufficiently large so as to have the capability to break up any particles that might become lodged in the spool valves causing them to jam up. The lack of mechanical coupling requires that four servo valves must be used in order to achive the reliability of a quad-redundant system (such as used on the shuttle).
Furthermore, due to the use of tolerance prone mechanical linkage between the torque motor and the spool, adjusting the null point of the spool is difficult to achieve. This could create a major problem if line-to-line type spool valves are used. Additionally, permanent magnet torque motors as disclosed by Glaze et al. are limited in output by iron saturation at the pole face; and thus, if high output force is required, the torque motor becomes quite large and creates significant packaging and weight problems. Another problem with this type of torque motor is that there is no practical way of adjusting the mechanical null point and thus, the adjustment must be made by mechanisms in the servo valve.
Other attempts at direct drive use such devices as voice coils to drive the spool. But use of voice coils requires a rather complex mechanical linkage system between the linear spool valve and coil, and thus difficulties are also experienced in establishing the null point for the spool. This is particularly a problem in dual type system where dual spools and dual voice coils are used.
Therefore, it is a primary object of this invention to proivde a simplified, lightweight, direct drive actuator system.
A further object of this invention is to provide a direct drive actuation system for controlling a control surface of an aircraft that can achieve reliability equal to conventional quad-redundant actuators.
A still further object of this invention is to provide a direct drive actuation system for controlling a control surface of an aircraft that integrates the servo valves and the servo valve drive mechanisms into the body of the actuator.